There have heretofore been described processes for producing a pinhole-free electrolytic copper foil for printed wiring boards in Japanese Patent Application Publication Gazette No. Hei 3-1391 (or 1391/1991) and Japanese Patent Application Laid-Open Gazette No. Hei 1-198495 (or 198495/1989).
However, an electrolytic cell having an electrolyte containing a certain concentration of copper ions, a cathode surface which moves while immersing the surface thereof into said electrolyte, and an anode surface installed to a position opposite to said cathode surface has been used in the art of producing a copper foil according to Japanese Patent Application Publication Gazette No. Hei 3-1391 (or 1391/1991). In a first zone through which the cathode surface passes in the electrolytic cell, copper nucleuses are formed on the surface of the cathode by applying a pulsed first current density pulsating with values greater and smaller than that of a limiting current density of copper ion. Subsequently, in a second zone through which the cathode passes in the electrolytic cell, a relatively smooth deposit of the copper foil is formed on the surface of the cathode by applying a current density smaller than the density of the limiting current density. Further, in a third zone through which the cathode passes in the electrolytic cell, a plurality of nodules are formed on the deposit of the copper foil by applying a pulsed second high current density pulsating with values greater and smaller than that of the limiting current density. The prior art according to Japanese Patent Application Publication Gazette No. Hei 3-1391 (or 1391/1991) is intended to produce a surface treated copper foil by giving a surface treatment including the process steps as stated above.
More specifically, the art of producing a copper foil according to Japanese Patent Application Publication Gazette No. Hei 3-1391 (or 1391/1991) is intended to form a highly pore free ultra-thin copper foil having an adhesive nodularized outer surface. Since a layer having the nodules is formed on an electroplated metal, however, there is provided at least one zone having a current density greater than the limiting current density in an electrolytic cell. This current density zone is formed by a processing anode provided via a gap or an insulating material completely separated from a primary anode and is provided at the outlet or inlet and the outlet of an electrolytic cell.
In the art of producing a copper foil according to Japanese Patent Application Publication Gazette No. Hei 3-1391 (or 1391/1991), however, the copper foil has been so arranged that the first anode is placed in the electrolytic cell and is set lower than the liquid level therein but the process anode is not located in a position opposite to the electrodeposition starting zone of the cathode surface, that is, opposite to the cathode surface in the vicinity of a surface of an electrolyte. The current density is lower than that in the cathode surface situated opposite to the first anode and a sufficiently high current density is not obtainable there. Therefore, the copper foil obtained are such that a number of nucleuses are not formable initially and satisfactorily. As a result, the aforementioned art has not succeeded in solving problems to be solved by this invention intended to provide a copper foil substantially free from curls and pinholes.
The art disclosed in Japanese Patent Application Laid-Open Gazette No. Hei 1-198495 (or 198495/1989) is intended to carry out electrolysis with an electrolyte not containing gas at the initial and final stages of electrodeposition in order to obtain a pore-free copper foil by discharging the spent electrolyte containing a large quantity of gas generated by the electrolysis from a submerged liquid outlet provided in the upper portion of an electrolytic cell. Since the anode is placed under the liquid level even in this case, the art disclosed therein, like Japanese Patent Application Publication Gazette No. Hei 3-1391 (or 1391/1991), has not also succeeded in solving problems to be solved by this invention.
The copper foil produced by the aforementioned prior art methods has internal strain and pinholes to varying degrees and the aforementioned prior art aimed at solving these problems still fails to attain its object.
There has been a recent tendency to make a copper foil thinner for use in printed wiring boards and has been developed a demand for a copper foil free from internal distortion and pinholes. The internal distortion of a copper foil in particular develops as a curl phenomenon, which is recognized from the fact that the edges portion of a copper foil turns up when it is placed on a flat table, for example. The number of pinholes and curls of a copper foil for printed wiring boards tends to increase as its thickness is decreased and this has posed a serious problem as the demand for a thinner copper foil is increased.
When copper foil lamination is automatically conducted by means of robots, curls of the copper foil produced by the prior art methods tend to make the robot commit an error in handling the copper foil for printed wiring boards, that is, make the robot fail to take hold of it; the problem is that the production of printed wiring boards is not smoothly carried out. Therefore, a copper foil substantially free from curls has been desired.